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1 Answer
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it's name of logical role not any particular configuration of particular operating system. it means that infected computers connect to such server periodically and download instructions what to do.

in case of typical botnets c&c server can respond with:

list of urls and time periods when infected computers should start DDOS attack

content of the mail messages and list of recipients - that's where spam you receive comes from

binary files that will be run locally on infected computers - this allows 'upgrade' of infected machines with new functionality [eg new malicious software that can scan the internet in search of more vulnerable computers].